Author Topic: AD4630-24 new SAR ADC from Analog Devices  (Read 19012 times)

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Offline fluxgate

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #25 on: January 18, 2022, 10:35:03 am »
Sorry, I did not want to say, that you can't get a clear fft out of SAR ADCs or that they are not useful for low frequency stuff.
In comparison to the AD7177 for example, these chips have a wildly different aliasing behavior.

The AD4630 has a -3dB Bandwidth of ~70MHz, this means that it heavily relies on Analog Filters and EM shielding, otherwise digital filtering gives you exactly nothing in the passband.
Of course you can get a very impressive Performance with this ADC, however it's not that forgiving compared to the AD7177.

So, I see no sense in using this chips to measure 1/f noise, while there are better suited Delta Sigma ADCs around.
 

Offline Kleinstein

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #26 on: January 18, 2022, 11:08:56 am »
Chances are the AD7177 would also react from aliasing in the 70 MHz band, maybe a little less than the SAR type chip, but not that significant. Both need AA filtering. The main difference is that with the SD chips the AA filter is usually just passive RC and can have a relatively low cross over. With a fast SAR ADC one usually wants the AA filter to start later (e.g. like at 500kHz) and than would need a steeper filter. When using oversampling and a lower frequency for the AA filter the difference is not that large. Both chips need a careful layout and a good (fast) driver.

The main point for the AD7177 is the easier to solder footprint and likely slightly lower price.
 

Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #27 on: January 19, 2022, 04:14:11 pm »
Bit more data attached. It would be interesting to see if the eval board shows the same effect.
 
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Online tszaboo

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #28 on: January 19, 2022, 04:36:00 pm »
You absolutely need to drive SAR ADCs with low impedance drivers/opamps. There is a switched capacitance on the input, and you need to be able to charge it within a fraction of the sample time within half of the LSB (step response). So you have a very high speed driver in front of it, otherwise you have nasty INL errors. They put a ADA4896 as example, an OPAMP with the GBW of 230MHz for a reason. You can also kiss goodbye to the DC accuracy if you use this driver, offset voltage is 500uV. And then you need to make sure about aliasing, drive the reference pin with the same care, as an input (because it is an input) and other issues. And the fact, that you have no way to analog debug your system because nobody makes tools to see 1 LSB errors in a 20+ bit system even at 1 MHz.
 

Offline KT88Topic starter

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #29 on: January 19, 2022, 07:14:27 pm »
@macaba: it looks like some digital- or RF signals create alias signals in the off-chip averaging example. It would be interesting to see your layout...

@tszaboo: Yes, SAR converters are demanding when it comes to the input driver. A shorted input however qualifies as low impedance source, if not screwed up in some way.
The AD4630 features a precharge cycle which charges the sample cap to the previous sample (p.20). As long as the signal frequency stays low, the drive requirements are quite relaxed. However, the data sheet still recommends fast amplifiers (p.23).
 

Offline Kleinstein

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #30 on: January 19, 2022, 07:53:15 pm »
The demands on the driver are not that much lower for the very low noise SD ADCs. These also sample the input with a capacitor at quite some rate. Chances are that in both cases the actual INL depends also on the driver and the reference driving part.
 

Offline fluxgate

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #31 on: January 19, 2022, 08:11:17 pm »
@tszaboo

For highspeed 20+ Bit Systems you can use fft and pure sine sources.
That way you can easily and very fast detect bit errors and verify overall system Performance.
The most difficult part is to find use cases for these systems.
 

Offline KT88Topic starter

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #32 on: January 19, 2022, 09:09:06 pm »
@Kleinstein:
This ADC frontend is different to previous ones. So far the ADC driver had to provide the full charge to the sample cap, starting from zero. This sample cap will be restored to the state of charge of the previous sample - the driver has to provide only the difference. For slowly changing signals this means a substancial reduction in driving current.
The downside is that the situation changes for mux'ed systems and high frequency, large amplitunde signals - the ADC would still work though....
 

Offline fluxgate

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #33 on: January 20, 2022, 08:54:18 am »
If you want to measure DC you can use a Big (~5uF) filter Capacitor in front of the ADC, then you don't need a highspeed opamp at all. Just look at the ltc2508 eval board. If I remember correctly that's how the DC specifications are measured.

I really don't think the AD7177 has the same aliasing issues, as it's intended for much lower frequency, it can have better internal filter. And from the needed Clock Frequency (16 MHz) I'm just gonna assume that it has higher internal sampling Rate.
If you take a look at the AD7177 eval board and Datasheet, they do not care about aliasing, Eval Board RC Filter Frequency is ~35MHz.

However I'm no Delta Sigma Expert and it's been a while since I've had the AD7177 at Hand, so it would be nice if someone with an eval board and signal generator would try it out.
 

Offline MiDi

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #34 on: January 20, 2022, 09:58:06 am »
Bit more data attached. It would be interesting to see if the eval board shows the same effect.



Thanks macaba :-+
On the timeseries for Manual avg (grey) one can spot some drift upwards.
Interesting that Manual shows some 1/f noise, maybe related to the drift.
Any idea where the peaks ~11Hz & 22Hz originate from?
How do you extract rms voltage noise frequency density from timeseries?
How does your test setup look like: metal case, SPI isolator, battery powered PSU?
« Last Edit: January 20, 2022, 10:15:56 am by MiDi »
 

Offline KT88Topic starter

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #35 on: January 20, 2022, 11:35:12 am »
It would be interesting to see the layout. The data look like a result of a split-ground setup which usually cuases digital noise to interfere with the ADC core... just a hunch.
 

Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #36 on: January 20, 2022, 12:07:34 pm »
On the timeseries for Manual avg (grey) one can spot some drift upwards.

Yeah, makes me wonder what the auto-zeroing inside the ADC is actually doing. Also - the constant offset between the two traces?! Maybe still beneficial to have mux in the front end to do occasional zero measurement.

Any idea where the peaks ~11Hz & 22Hz originate from?

Nope. I suspect that in a run of 32768 samples, there will be a sample that is a massive deviation that drags it down/up. Note: I used the ADC test mode (where it outputs a constant value) to check that it isn't a glitch on the flexSPI readback.

How do you extract rms voltage noise frequency density from timeseries?

I have an NSD estimator algorithm that I have validated previously (to ensure the amplitude is "correct"). Uses multi-taper spectral estimation with discrete prolate spheroidal sequences (yes, a mouthful). It provides slightly better spectrums for the same amount of input data when compared to the traditional Welch method, not really enough of an improvement to justify it, it was just an intellectual curiosity for me.

How does your test setup look like: metal case, SPI isolator, battery powered PSU?

No metal case (tried it in my shielded steel experiment box, no change), no SPI isolator (only level translator), USB to laptop (switching laptop between AC and battery didn't make a difference). The analog supplies are powered from 9V battery, digital supplies from USB connection.

The data look like a result of a split-ground setup

Single ground-plane. As this is a BGA package, every ground pin goes through via to ground plane whereas I note, with interest, that the eval board uses solid copper planes directly under the BGA. I may try that, despite being against BGA-layout convention (would also mean double copper thickness compared to plane on internal layer). BGA fanout attached for your intellectual curiosity.
« Last Edit: January 20, 2022, 12:11:26 pm by macaba »
 
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Offline KT88Topic starter

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #37 on: January 20, 2022, 12:18:02 pm »
Thanks for sharing the layout. It looks like the groundplane doesn't continue underneath the digital interface traces. Do you use a different groundplane for the micro/FPGA?
 

Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #38 on: January 20, 2022, 12:29:45 pm »
Thanks for sharing the layout. It looks like the groundplane doesn't continue underneath the digital interface traces. Do you use a different groundplane for the micro/FPGA?

Ground plane is the dark red background, covers the whole board. The green(?) is power supplies/ref. RP2040 micro is off-board (on the "Pico" dev board) with relatively poor ground connections (single ground wire). Now that I've confirmed the RP2040 PIO to be a perfect match for this ADC, I'll probably do v2 with onboard RP2040.
 

Offline miro123

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #39 on: January 22, 2022, 09:31:45 pm »

A. 2MSPS, microcontroller averaged x32768 = 2uVpp
B. 2MSPS, onboard averaged x32768 = 0.7uVpp
Adds a little challenge for me - how to get 1PLC of samples with only the 2^n onboard averaging options.

Are you using STM32, if yes you can consider using stm32H7 - their AXI RAM is up to 512KB
 

Offline TiN

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #40 on: January 22, 2022, 10:00:09 pm »
If you can buy it :-X Plane under BGA often used for higher power devices acting as heatsink too.
YouTube | Metrology IRC Chat room | Let's share T&M documentation? Upload! No upload limits for firmwares, photos, files.
 

Offline CurtisSeizert

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #41 on: February 26, 2022, 05:20:59 am »
Bit more data attached. It would be interesting to see if the eval board shows the same effect.

I just got things working with the eval board. The eval software times out on long acquisitions, so I could only get 800 samples for an fft.  This was taken with shorted inputs.  Sampling rate was 2 Msps, block size=32768.  The fft x-axis scaling seems to be calculated without taking into account sample averaging, so you need to divide by 32768 to get the actual frequency (divide by 65536 for the fft with that block size).  The "peak" with a dot above it corresponds to about 13 Hz.  The output is 30 bit codes, so peak to peak in this trace is about 1.3 uV over 13s with an offset of -10.6 uV.  With block size of 65536 and 400 samples, peak to peak is just under 1 uV.  The fft peak it labels as the fundamental there is 1 Hz.

Curtis
« Last Edit: February 26, 2022, 02:31:03 pm by CurtisSeizert »
 
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Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #42 on: February 26, 2022, 10:06:19 am »
Thanks for joining in, it's good to get your input.

I see from "AD4630-24 trace.JPG" that it's 130 counts peak-peak, with offset of -1140 counts. I calculate that as being 1.2uVpp with 10.6uV offset (N*(10/2^30)). We are both getting values better than the datasheet value of 1.8uVpp.

Does the input data into the FFT have the mean subtracted to reduce the effect of DC? It's a shame the software isn't allowing longer captures to get a cleaner spectrum.
 

Offline KT88Topic starter

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #43 on: February 26, 2022, 02:17:19 pm »
Better results than secified in the DS are no surprise for such a high DR ADC. Testing to such a high degree of certainty would come at painful price premium...
 

Offline CurtisSeizert

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #44 on: February 26, 2022, 02:41:26 pm »
Thanks for joining in, it's good to get your input.

I see from "AD4630-24 trace.JPG" that it's 130 counts peak-peak, with offset of -1140 counts. I calculate that as being 1.2uVpp with 10.6uV offset (N*(10/2^30)). We are both getting values better than the datasheet value of 1.8uVpp.

Does the input data into the FFT have the mean subtracted to reduce the effect of DC? It's a shame the software isn't allowing longer captures to get a cleaner spectrum.

You are right. I did not multiply by the FS voltage.  I knew those numbers looked too good to be true, especially with the board flapping in the breeze.  I will see if I can export the raw data from the capture to perform the fft myself - it is not clear how that spectrum is calculated in the software.  I will see if I can get around the USB timeout for a longer capture.
 

Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #45 on: June 27, 2022, 12:34:38 pm »
A. 2MSPS, microcontroller averaged x32768 = 2uVpp
B. 2MSPS, onboard averaged x32768 = 0.7uVpp

EDIT: I'm now reasonably certain this is an issue with layout rather than any deficiency with the part itself.

Quick update, using fresh AD4630 on v2 PCB with improved layout.

A. 2MSPS, microcontroller averaged x32768 = 0.62uVpp
B. 2MSPS, onboard averaged x32768 = 0.65uVpp
(10 second sample block [i.e. 30.5 - 0.1Hz] peak-peak values averaged over 15 minutes [90 blocks])

Improved layout has solved the additional noise issue I was seeing. It's interesting that the microcontroller averaging is slightly better (it's a very stable 2 decimal places, no margin of error issues here), I suspect it's due to the loss of bits in the onboard averaging filter.

Now to move onto other interesting tests and NPLC averaging lengths...
« Last Edit: June 27, 2022, 12:36:34 pm by macaba »
 
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Offline MiDi

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #46 on: June 28, 2022, 05:27:30 am »
Little brother with 1CH is out: AD4030-24
 
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Offline macaba

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #47 on: June 28, 2022, 03:21:53 pm »
Little brother with 1CH is out: AD4030-24

Interesting!

I think as it's the same package, might as well use the 2 channel version - a single-ended front end to avoid the difficulty of getting a differential resistor matching >140dB (one of the major challenges of HPM7177) is easiest, and having the 2 channels in parallel from the same input gains back dynamic range where it is lost by not having fully differential front end.
 

Offline Kleinstein

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #48 on: June 28, 2022, 03:41:02 pm »
I don't think one need good resistor matching for a differential front end. Ideally the 2 sides are matched, but if not this mainly reduces the input ranges a little. Normally one does not care about things like 1% of the full scale range lost or 0.01% higher noise from matching to only 1%.
 

Online iMo

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Re: AD4630-24 new SAR ADC from Analog Devices
« Reply #49 on: June 29, 2022, 05:36:05 pm »
Is there a feasible option other than the LT5400 for the input dividers with SAR/SD ADCs (differential inputs)? I've built myself a simple 10V voltmeter (w/ 24bit SD, single 10V input) and I do not see any feasible variant than a floating opamp + LT5400-8 9k/1k.. Currently I have there an old noname thick film 1:10:100:1000.. divider, but its TC is something like 70ppm/C..  :D
« Last Edit: June 29, 2022, 05:56:30 pm by imo »
 


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